In recent years, there has been considerable interest in the photo-, e-beam and thermally induced cationic polymerization of many types of monomers and oligomers. Such polymerization systems are currently employed in a wide diversity of industrial applications including: non-stick release coatings, adhesives, abrasion resistant coatings for plastics, optical fiber coatings, reinforced composites and optical waveguides. Considerable future potential for growth of UV curable, cationically polymerizable systems exists because of the excellent properties of such coating systems, their low energy consumption and the lack of environmental pollution which derives from the elimination of the use of solvents.
Key to this technology was the development of highly photosensitive (i.e. high quantum yield) cationic photoinitiators which can be designed to be responsive to various UV wavelengths. Among the best photoinitiators which have been developed in recent years are diaryliodonium and triarylsulfonium salts of the structures I and II respectively, indicated below. ##STR2## In photoinitiators I and II, A.sup..crclbar. is a non-nucleophilic anion such as CF.sub.3 SO.sub.3.sup..crclbar., ClO.sub.4.sup..crclbar., FSO.sub.3.sup..crclbar., SbF.sub.6.sup..crclbar., BF.sub.4.sup..crclbar., PF.sub.6.sup..crclbar., AsF.sub.6.sup..crclbar., (C.sub.6 F.sub.5).sub.4 B.sup..crclbar., CB.sub.11 H.sub.6 Cl.sub.6.sup..crclbar., and CB.sub.11 H.sub.6 Br.sub.6.sup..crclbar..
While I and II exhibit high quantum yields, they are only soluble in the most polar of cationically polymerizable monomers. Further, both of these photoinitiator classes are toxic. These two factors considerably limit both their attractiveness and their utility. In an effort to alleviate these problems, photoinitiators III and IV, bearing C.sub.(1-20) alkoxy groups, V, bearing two alkyl groups, and VI, bearing both an alkoxy and a hydroxy group, were developed. ##STR3## These modifications were largely successful in rendering the respective salts non-toxic and soluble in a broader range of monomers.
An additional class of interesting photoinitiators, the dialkylphenacylsulfonium salts, having the general structure VII shown below, were described in the early 1980's: ##STR4## In the compounds described, Ar was commonly phenyl, R.sup.2a, was methyl and R.sup.3a was methyl, or R.sup.2a and R.sup.3a taken together formed a 5 or 6-membered ring. These compounds were useful cationic photoinitiators with certain classes of monomers, but they have never enjoyed the commercial success of the triarylsulfonium salts.
There remain considerable difficulties with the diaryliodonium and triarylsulfonium photoinitiators III-VI. While photoinitiators III-VI have better solubility characteristics as compared to I and II, they are still poorly soluble in non-polar monomers such as epoxidized hydrocarbon polymers, multifunctional vinyl, 1-propenyl and 1-butenyl ethers and in epoxy silicone oligomers employed for release coatings. Similarly, the dialkyl phenacylsulfonium photoinitiators VII generally display very poor solubility in these same monomers, as a result of which UV cure of the monomers and resins is considerably below optimum. Poor and erratic cure are observed in some cases and no cure in others. Moreover, in the case of those photoinitiators that are produced as a mixture of isomers, such as V, purification by conventional crystallization techniques is generally unsuccessful. As a result, formulations of these photoinitiators with reactive monomers and oligomers exhibit poor potlife, having a very pronounced tendency to undergo rapid and exothermic gelation on standing. This is highly undesirable. There is an unmet need for truly soluble, compatible, highly reactive, shelf-stable cationic photoinitiators. The instant invention satisfactorily resolves the problems outlined above with existing cationic photoinitiators.